The basic structure of a capacitive acoustic transducer is that of a conductive membrane suspended above a conductive electrode. When a voltage is applied between the membrane and the electrode, coulombic forces attract the membrane to the electrode. If the voltage applied varies in time, so too will the membrane position, emitting sound at the frequency of the voltage modulation (membrane deflection). Conversely, acoustic vibrations will set the membrane in motion, and the resulting capacitance variation can be detected electrically. A capacitive acoustic transducer can consist of one or many such membranes. A sketch of a single membrane acoustic transducer is shown in FIG. 1. A transducer consists of many such elements. Each transducer element includes a conductive substrate 11 such as a silicon. A membrane 12, which may be silicon nitride, is supported above the substrate by an insulating support 13, such as silicon oxide. A conductive film or layer 14, such as gold, forms a conductive electrode. The film and silicon substrate form the electrodes of the transducer. The transducer may be a sealed transducer with the space 16 evacuated.
Co-pending application Ser. No. 08/792,114, filed Jan. 31, 1997, describes microfabricated electrostatic ultrasonic transducers; co-pending application Ser. No. 08/739,446 filed Oct. 29, 1996 describes broadband ultrasonic transducers and a method of fabrication; and U.S. Pat. No. 5,894,452, issued Apr. 13, 1999 describes an immersion ultrasonic transducer and method of fabrication. The foregoing applications are incorporated herein in their entirety by reference.
It is known that the shape of the membrane(s), in combination with the material properties of the membrane(s) and the medium, will determine the transducer's resonant frequency and quality factor. In co-pending application Ser. No. 08/739,446 the membrane shape was determined by the location of the access holes or vias which allowed etchant to remove selected portions of a sacrificial layer. Errors from the desired geometry can result because of variations of etchant activity (temperature, shelf life, contamination, etc.) and etching time. Furthermore, complex shapes are only approximated, not precisely defined. It would be advantageous to provide a method of microfabricating an acoustic transducer in which the membrane shape and geometry is precisely defined, the membrane is not perturbed by vias needed for the sacrificial etching and in which vacuum sealing can be achieved with a wide variety of chemical species.